A compound represented by the following structural formula:

that is, 7-[(1R,5S)-1-amino-5-fluoro-3-azabicyclo[3.3.0]octan-3-yl]-6-fluoro-1-[(1R,2S)-2-fluorocyclopropan-1-yl]-1,4-dihydro-8-methyl-4-oxoquinoline-3-carboxylic acid (hereinafter, referred to as compound A), has wide and strong antibacterial activity against Gram-positive bacteria, notably against resistant Gram-positive cocci such as multiple resistant pneumococcus including quinolone resistant, and against Gram-negative bacteria. Compound A also has such excellent safety that it causes only with low probability previously known side effects of antibacterial agents of this class such as convulsion induction and phototoxicity (photosensitivity) and recently clinically reported side effects such as cardiotoxicity (QT prolongation), blood glucose level abnormality, and delayed rash. It has also become clear that compound A shows excellent oral absorbability and permeability into organs. Thus, compound A is expected to be an excellent antibacterial agent (Patent Literature 1).